Be an Early Applicant
The Role
Founding engineer will design, build, and operate core systems: order book, execution stack, exchange connectivity, and portfolio management. Work spans backend, frontend, data pipelines, and low-latency integrations. Define architecture, own production reliability and observability, conduct code reviews, and help shape engineering culture and early hiring.
Summary Generated by Built In
About River Markets
I’m Antonin, co-founder and CTO of River Markets. Oscar (CEO) and I spent 3 years as quants in HFT shops and hedge funds. At nights and weekends we started trading predictions markets for fun and built a ton of trading infrastructure.
We are now using this to build the prime brokerage and execution layer for prediction markets: the infrastructure professional and institutional traders need to access, trade, and manage risk across venues like Kalshi and Polymarket at scale.
We value ownership, speed, and a high technical bar. We expect to do more with fewer people than a team our size has any right to. We are currently fully connected to 3 exchanges, have 6 proprietary execution algorithms and already have paying users.
How we work: AI-native
AI is the operating model. Every engineer treats AI as a core part of how they work: writing and reviewing code with AI tooling, automating their own workflows, and building internal systems that lean on LLMs where it makes sense. The bar isn't "can you use Claude": it's "what did you stop doing by hand this week?"
We're looking for people who instinctively reach for automation, who multiply their output rather than just grinding harder, and who are excited that the leverage per engineer here is genuinely uncapped. We expect engineers to still understand every line they ship, AI-generated or not.
What you'll do
As a founding engineer, you'll own core systems end to end, scoping features directly with the founders, designing the architecture, building backend and frontend, integrating with exchange APIs, and watching it run in production. Concretely, you'll:
· Build and maintain the systems behind our order book, execution stack, exchange connectivity and portfolio management.
· Integrate with external exchanges and APIs where reliability and latency matter
· Write high-quality, testable code and raise the bar on everyone else's through review
· Improve infrastructure, reliability, and observability so we can move fast without breaking our systems
· Help define our engineering culture, standards, and early hiring, you're setting the defaults the next ten engineers inherit
We expect the following:
· By week 1, you’ll have shipped in production a feature real traders use.
· By month 1, you’ll own a core system end to end, from architecture to monitoring.
· By month 3, you’ll have shared our engineering culture and be involved in our next hire
What we're looking for (we don’t expect you to check all the requirements!)
- Strong engineer (at least 2 YOE -full time- in SWE or similar role) who has designed, shipped, and operated production systems, not just written features, but kept them alive.
- Fluency in a modern language (Python, Go, Rust, or similar) and comfort across the stack: backend services, data pipelines, and user-facing components
- Solid grounding in system design, API design, and data modeling for low-latency, scalable applications
- AI-native by default, you already automate your own work and use AI tooling to ship more, faster
- Thrives in a small, high-ownership environment with minimal oversight
- Someone nice (we’ll spend a lot of time together) :)
Bonus: experience in fintech, trading, or market infrastructure; a genuine interest in prediction markets
Compensation, Benefits & Perks
- Base salary: $140,000–$200,000
- Equity: 0.5%–1.5%
- On-site in the San Francisco Bay Area
- Platinum health insurance plan, we cover 100% of the premium
- Unlimited PTO (minimum of 3 weeks, we don’t want a burned out team)
- Food provided in office
- Dog Friendly office
- Fresh fruits + snacks from farmer markets and local shops
- Monthly team building activity
- 401(k) match fully vested
- We sponsor visas.
The base pay range for this role is $140,000 – $200,000 per year.
Skills Required
- At least 2 years full-time software engineering (SWE) experience
- Designed, shipped, and operated production systems (including keeping them live)
- Fluency in a modern language (Python, Go, Rust, or similar)
- Comfort across the stack: backend services, data pipelines, and user-facing components
- Solid grounding in system design, API design, and data modeling for low-latency, scalable applications
- AI-native: automates own work and uses AI tooling to increase productivity
- Thrives in a small, high-ownership environment with minimal oversight
- Good interpersonal skills / someone nice to work with
- Experience in fintech, trading, or market infrastructure
- Genuine interest in prediction markets
Am I A Good Fit?
Get Personalized Job Insights.
Our AI-powered fit analysis compares your resume with a job listing so you know if your skills & experience align.
Success! Refresh the page to see how your skills align with this role.
The Company
What We Do
Midwestern Interactive, LLC is a U.S.-based software and product development firm that delivers cloud and mobile applications, digital strategy, and creative/media services. The company provides scoped projects, embedded engineering teams, and recruiting/talent services to help clients accelerate product development and scale technical capability. Founded in 2012 and headquartered in Joplin, MO, Midwestern focuses on end-to-end product delivery and talent partnerships for growing businesses.
